1. Field of the Invention
The present invention is directed to a method and system for the process of reactively converting a liquid fuel into a gasified stream. The method and system of the present invention also provide a means for quickly starting and operating the process at a variety of inlet conditions including ambient inlet conditions. More particularly, the method and system include the steps of atomizing a liquid fuel, mixing the atomized fuel with an oxidizer in fuel rich proportions, partially vaporizing the fuel and passing the resulting stream through a catalyst bed. The catalyst bed can be suitable for supporting partial oxidation/reforming reactions at high space velocity and can be integrated with an ignition source to initiate the reaction. One embodiment results in an integrated system comprised of a fuel injector, mixer, vaporizer, catalytic-reactor, and igniter, and represents a simplified fuel preparation/gasification and reformation approach reducing the predisposition to coking, especially with liquid fuels.
The method of the present invention can be used in applications where gasification of long chain hydrocarbons is required or desired to improve performance. These pre-reforming reactors can be integrated directly with fuel cells, IC engines or burners. For example, soot and coke are often byproducts of long chain hydrocarbon combustion, such as in diesel engines. Soot is formed when liquid droplets of diesel fuel are combusted prior to vaporization. Application of this method will allow pre-vaporization and partial pre-reforming of diesel fuel prior to combustion. Pre-reforming, as used herein, describes the stage of reforming of a hydrocarbon fuel prior to combustion of that fuel, as described herein, in comparison to some other mechanism that may commonly be referred to as reforming of a fuel.
The integrated fuel injector/mixer/vaporizer/catalytic-reactor/igniter allows pre-reforming liquid hydrocarbon fuels without the need for external pre-heat or complete pre-vaporization. The reactor permits a simplified fuel injection, mixing and vaporization approach reducing the predisposition to coking when using such fuels by taking advantage of a catalytic reaction and rapid mixing within the catalyst bed. One such catalytic reactor comprises catalytically coated, short-contact-time, ultra-short-channel-length substrates such as those disclosed in U.S. Pat. No. 5,051,241; the contents of which are incorporated herein by reference, particularly the teachings at Column 2, line 47 through Column 4, line 55. One such means is commercially available from Precision Combustion, Inc., as Microlith™. This technology provides the ability to impede soot formation kinetics, increase diffusive capabilities to boost mixing, and provides high surface area to augment vaporization.
2. Brief Description of the Related Art
Vaporization of liquid fuels (e.g., alcohols, hydrocarbons) typically is achieved by indirectly supplying heat into a stream of liquid fuel via heat exchange with a hot wall. One disadvantage of this method is that the rate of vaporization is limited by the rate of heat transfer such that relatively large surface area is required for fuel vaporization. Another disadvantage of this method, especially for vaporizing long chain hydrocarbons, is that heating the fuel stream to the vaporization temperature tends to cause fuel decomposition and formation of deposits. More specifically, coke formation is problematic. Moreover, preventing deposits from forming within the fuel passages in the nozzle during steady state operation due to heat-up of the nozzle from the downstream hot zone is challenging.
Another known method for gasification of a fuel stream comprises mixing atomized fuel with a hot gas such as superheated steam that supplies the heat required for fuel vaporization and prevents coke formation. However, the large amounts of superheated steam required in this method result in a large heat load for steam production.
Spray methods for atomization of liquid fuels known in the art include air-blast or pressure atomizers, ultrasonic and electrospray atomizers. These spray systems are capable of providing a uniform distribution of atomized fuel across the entrance of the catalyst bed. Such atomizers may include a co-flow of air that allows mixing of the fuel and oxidizer. However, very fine and uniform droplet size along with homogeneous fuel-air distribution, required to avoid coke formation and obtain temperature/mixture uniformity in the reactor, is difficult to achieve in practical systems.
Ignition devices, such as a spark or glow plugs, are widely used to ignite fuel-oxidizer mixtures at startup. These devices often are subject to failure due to the high operating temperatures by virtue of their location required for ignition.
Monoliths are commonly used catalyst substrates for the gasification of liquid fuel. Fuel oxidizer mixture inhomogeneities are usually detrimental to these substrates as they lead to localized lean or rich zones respectively causing hot spots or carbon precipitation regions. Since there is no opportunity for these zones to re-mix within the long, separated channels of a monolith, these substrates are particularly vulnerable. In addition, carbon precipitation is favored in monoliths due to the boundary layers that develop in these substrates.
Combustion of liquid fuels in fuel cell or internal combustion engine systems poses significant problems, especially for fuels with high aromatic content and wide boiling point distribution. This can be attributed to the propensity of the heavier aromatic compounds in the fuel to form deposits or coke when vaporized at high temperatures. Accordingly, there is a need for a pre-reforming reactor capable of operating with a range of liquid fuels. It is therefore an object of the current invention to provide a pre-reforming reactor for partially oxidizing and cracking the heavy components of the fuel. The pre-reformed fuel subsequently can be further reformed or combusted to power fuel cell systems, internal combustion engines, burners, and other known devices.
U.S. Pat. No. 4,381,187 to Sederquist (the “'187 Patent”) discloses a method in which a partially pre-vaporized fuel stream mixed with air, at an overall equivalence ratio greater than 3, is passed through a monolith catalytic structure thereby achieving gasification of the fuel in the stream. The '187 Patent requires mixing the fuel stream with the heated air stream and partial vaporization of the fuel prior to its introduction into the catalyst bed. Air temperature specified for the method is between 580 and 660° C. Thus, coking may occur. The method of the '187 Patent therefore requires supplying external heat for pre-heating the air. The method of the '187 Patent also requires the catalyst to be in a shape having wall surfaces extending in a downstream direction defining a plurality of parallel cells, for example, a conventional monolith. This configuration results in a comparatively low conversion rate of the reactants to the desired products. Moreover, in the method of the '187 Patent, the catalyst is chosen such as to initiate and sustain complete combustion, namely oxidation of part of the fuel to CO2 and H2O releasing heat. The '187 Patent discloses at column 1, line 49, that “once in vaporous form, fuel may be catalytically partially oxidized and reformed in an autothermal catalytic reactor.” Therefore, a separate reactor is required if H2-rich gas stream is desired.
It is therefore another object of the current invention to provide a catalyst substrate that facilitates mixing of the stream flowing therethrough, for example a substrate having plurality of voids in random order and short channels extending in the downstream direction the length of which is similar to the channel diameter. Such a configuration results in a comparatively high conversion rate of the reactants to the desired products and minimizes break through of unreacted fuel. It also is an object of the current invention to provide a catalytic reactor for the gasification of liquid fuels comprising a catalyst that yields partial oxidation products, such as CO and H2. This results in a higher level of fuel conversion for the same amount of added air and produces hydrogen-rich gas directly from the gasifier reactor. It is a further object of the current invention to provide a method whereby steam or atomized water and/or CO2 may be added to the fuel/air stream to adjust the amount of hydrogen in the product stream. It also is a further object of the current invention to provide a method whereby no external pre-heating of either air or fuel is required.
The dependence of fuel conversion on oxygen-carbon ratio (O:C) is known to one skilled in the relevant art. Tests of a conventional gasifier comprising a catalytic reactor and prevaporized, premixed fuel and air inlet indicated a linear increase in fuel conversion with increasing air. With increased air, or a higher O:C, the catalyst temperature increased and higher fuel conversion can be achieved, though at the expense of higher heat release and higher catalyst temperatures. It thus is a further object of the current invention to provide a method whereby gasification of liquid fuels is achieved employing a fuel-rich fuel air mixture with an O:C ratio suitable for efficient fuel conversion.